9299

• UCREFRP Catalog Number: 9299
• Author: Anderson, R. and J. Mumma.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual assessment of Colorado Pikeminnow Larval Production in the Gunnison and Colorado Rivers, Colorado 1992-1996.
• Copyright Material: NO

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subtracting the average incubation time of fertilized eggs at 18C (6 d, Bestgen, 1997) from
the estimated hatch date.

Total fish used in the analysis are larvae and YOY (fry) combined. In most years
most fish collected were larvae, but in 1994 most net samples were comprised of a large
percentage YOY as classified by Snyder (1995). Lengths of Colorado pikeminnow
young-of-year and larvae were tested for differences between years using the t-test, and
mean size were correlated to spawning dates, degree days and flow.

Data from two concurrent studies in Utah were compared to results of this study
to determine the influence of Colorado pikeminnow produced in Colorado on larval
abundance in the drift and in nursery habitats downstream in Utah. The report, "Annual
assessment of Colorado pikeminnow larval production in the Colorado River, Utah 1992-
1996" (Trammel and Chart, 1999a) was examined to identify longitudinal relationships
between drift abundance at Loma to two sites downstream in Utah. The report "Aspinal
unit studies: Evaluation of nursery habitat availability and Colorado pikeminnow young-
of-year habitat use in the Colorado River, Utah 1992-1996" (Trammel and Chart, 1999b)
was used to compared larval abundance in the drift to estimates of year-class strength
made by fall and spring backwater seining.

RESULTS

COLORADO COLLECTIONS

Larval Fish - Total Catch

A total of34,912 fish larvae and YOY were sampled in the drift over the course of
the study. Daily capture data for each species in the sample is included for the Loma and
the Middle Colorado River to indicate beginning and ending of drift season for each year
(Appendices 1, and 2). Season summary catch data for each station is given in Appendix
3. Loma consistently had the highest total catch ranging from 1,889 in 1994 to 6,321 fish
in 1995 (Appendix 1 Table 1). Either Upper Colorado River or Bridgeport had the lowest
catch in each of the five study years. More total native larval fish per effort were caught in
1995 than any other year and the highest catch rate for nonnative fish larvae was in 1992.
The lowest catch rate of native larval fish was in 1994 and 1996 produced the lowest
catch rate for nonnative fish larvae.

The timing of the catch was variable between the five study years. In 1995
summer flows were high, late and cold and larval catch was very late in that summer
compared to other years. In 1995 very few native or non-native fish were collected in
drift nets during the entire month of July and the catch rates peaked in mid August 1995
(Appendix 2, Figures 7 and 8). In 1994, a year with low, early and warm summer flows,
daily catch rates were highest in late June and few larval native fish were collected in
August (Appendix 2, Figure 5)